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For every 10 decibels of hearing loss, the extra risk increased by 20 percent.

Seniors with high levels of hearing loss are significantly more likely to develop dementia over time than those who retain their hearing, according to a new study by Johns Hopkins University and the National Institute on Aging published in the Archives of Neurology.

The new study, published in the February 2011 issue, focused on 639 people whose hearing and cognitive abilities were tested as part of the Baltimore Longitudinal Study on Aging between 1990 and 1994. The research was supported by the intramural research program of the National Institute on Aging.

While about a quarter of the volunteers had some hearing loss at the start of the study, none had dementia. Researchers also took other dementia risk factors into account, including diabetes, high blood pressure, age, sex, and race.

These volunteers were then closely followed with repeat examinations every one to two years, and by 2008, 58 of them had developed dementia, including 37 who had alzheimer’s disease. Participants with hearing loss at the beginning of the study were significantly more likely to develop dementia by the end. Compared with volunteers with normal hearing, those with mild, moderate, and severe hearing loss had twofold, threefold, and fivefold (respectively) the risk of developing dementia over time.

Of the participants, 125 had mild hearing loss (25 to 40 dB), 53 had moderate hearing loss (41 to 70 dB), and six had severe hearing loss (more than 70 decibels).

The risk of dementia was higher in those with hearing loss of greater than 25 decibels, with further increases in risk observed among those with moderate or severe hearing loss as compared with mild hearing loss. For participants age 60 and older, more than one-third of the risk of dementia was associated with hearing loss.

The risk of developing Alzheimer’s disease, specifically, also increased with hearing loss, such that for every 10 decibels of hearing loss, the extra risk increased by 20 percent. The results showed no association between self-reported use of hearing aids and a reduction in dementia or Alzheimer’s disease risk.

While the reason behind the link remains unknown, investigators believe a common pathology may underlie both, or that the strain of decoding sounds over the years may overwhelm the brains of hearing loss patients, leaving them at higher risk for dementia. Investigators also suggest that hearing loss could lead to dementia by making individuals more socially isolated, which is a known risk factor for various cognitive disorders, including dementia.

“Researchers have looked at what affects hearing loss, but few have looked at how hearing loss affects cognitive brain function,” says study leader Frank Lin, MD, PhD, assistant professor in the Division of Otology at Johns Hopkins University School of Medicine. “There hasn’t been much crosstalk between otologists and geriatricians, so it’s been unclear whether hearing loss and dementia are related.”